(a) Field of the Invention
The present invention relates to an automotive lamp assembly, and more particularly to a lamp assembly having a reflector specially designed to provide a light distribution pattern which permits to make the most of the rays of light emitted from a lamp bulb of the lamp assembly provided on a car for illumination of the road surface before the car.
(b) Description of the Prior Art
FIG. 1 shows an example conventional automotive lamp assembly having a lamp housing 1 and a reflecting surface 1a integrally formed on the inner surface of the lamp housing 1 and which takes the form of a paraboloid of revolution. A lamp bulb 2 is disposed near the focus of the reflecting surface 1a, and a front lens 3 is disposed covering the front opening of the lamp housing 1 and as fixed to the circumferential edge of the front opening of the lamp housing. The front lens 3 has prisms formed on the inner surface thereof which faces the lamp bulb 2. If the reflecting surface 1a has the geometrical form of a paraboloid of revolution, namely, if both the section, of the reflecting surface 1a, along the vertical plane in which the optical axis Z and that along the horizontal plane in which the optical axis Z also lies take the form of a paraboloid, all the rays of light a emitted from the lamp bulb 2 are so reflected at the reflecting surface 1a in the directions parallel to the optical axis Z as to be beams b nearly parallel to each other. The parallel beams b are so refracted by the prisms on the front lens 3 as to be diverged in such directions as indicated by arrows c1 and c2, finally forming a predetermined light distribution pattern. Most of the light distribution patterns and their luminous intensity distributions from the automotive lamp assemblies have a reflector of a type of which the reflective surface is geometrically formed by quadrics such as a paraboloid of revolution, ellipsoid of revolution or the like or a combination of such different curved surfaces depends upon the front lens 3. However such conventional reflectors are limited in luminous intensity in many cases. The conventional automotive lamp assembly whose luminous intensity depends upon the reflector, not upon the front lens, typically employs a compound-curvature reflecting surface as disclosed in, for example, the U.S. Pat. No. 3,492,474. FIG. 2 is a schematic horizontal sectional view of a so-called divergent-type reflector, by way of example, which has a compound-curvature reflecting surface of which the vertical section has a parabolic curvature while the horizontal section has a hyperbolic curvature. The rays of light emitted from the lamp bulb 4 are so reflected at the reflecting surface of the reflector 5 as to be parallel beams in the vertical plane and divergent beams in the horizontal plane, the latter beams being diverged away from the optical axis. The reflector of this example lamp assembly is provided on the circumferential edge of the front opening thereof with a transparent cover 6 having no prisms formed on the inner surface thereof and which thus covers the front opening. So this lamp assembly needs no special lens configuration. However, this lamp assembly is disadvantageous in that as the luminous intensity at the central zone of the light distribution pattern is increased, that at the peripheral zone decreases, while the luminous intensity at the central zone decreases when that at the peripheral zone is increased. Namely, the luminous intensity distribution in the light distribution pattern cannot be freely controlled. Further, for more effective utilization of the beams from the lamp bulb, it is necessary to design a relatively large area of the front opening of the reflecting surface, that is, a relatively large horizontal width of the front lens. Therefore, the reflector having the above-mentioned configuration cannot be adopted in a relatively small lamp assembly. These problems greatly limit the freedom of designing the automotive lamp assemblies.